Gibson what year? Early carbs formed a drop of gas on the idle port hole, it would get big and fall off creating one rich cylinder load then the drop would start building again. Thanks to 2M for revealing the SB about this. I think he problem was cured for good with the 73 TX650, somewhere around there.
other possibilities a leaky choke plunger seal, float level, float leak.
Might want to cross check the tach reading if your ear isn't used to the XS650 pulse.
Darkened garages can reveal erratic high voltage leaks also. spark plug caps, condensers, an inline spark gap can show up "missing" sparks.